Proton exchange membranes (PEMs) are used in fuel cells to electrically insulate an anode from a cathode and to conduct protons from the anode to the cathode during fuel cell operation. PEMs usually include a polymer matrix having attached functional groups which are capable of exchanging cations. The polymer matrix is typically an organic polymer material such as polystyrene, polytetrafluoroethylene (PTFE) or polytetrafluoroethylene analogs, or perfluorosulfonic acid (PFSA). Sulfonic acid groups are incorporated into the polymer matrix.
It is important for the PEM to have certain properties for optimum functioning of a fuel cell. These properties include a high ionic and no electronic conductivity, low gas permeability, resistance to swelling and high mechanical strength. Therefore, various attempts have been made to improve one or multiple properties of a fuel cell PEM. For example, inorganic particles have been impregnated into PFSA using sol-gel chemistry in order to improve water retention of the PEM at temperatures higher than 100 degrees C. Solid inorganic proton conductors, such as zirconium phosphate, have been introduced into PFSA resin via methods such as ion exchange followed by precipitation. In addition to high-temperature water retention capability similar to that offered by silicon oxide, the solid inorganic proton conductors impart additional proton conductivity to the PEM.
Other methods for enhancing properties of a PEM include the use of porous inorganic glass (SiO2—P2O5 made via sol-gel chemistry) as a high-temperature PEM. In that case, —P—OH groups bonded to the Si—O network provide the proton conduction mechanism. The inorganic glass is used as a bulk material rather than as a filler in a polymeric resin such as PFSA.
According to the present invention, an organic/inorganic hybrid composite proton exchange membrane includes a solid inorganic conductor which is used as filler in a PFSA resin. The solid inorganic conductor imparts enhanced water retention at high temperatures, as well as enhanced proton conductivity, to the PEM.